Actually, WalMart has the right paint. It is labeled Kids Paint, made by a major manufacturer and certified kid safe. It comes in primary colors and some others like green, orange and purple.

It is acrylic enamel and dries very bright and shiny.

I used it for painting several varities of toys I made for several years.

I'd also like the colors to be rich and saturated, but not opaque; I want the wood grain to show through.

By it's very nature, paint is opaque. Use a stain or dye and top coat with a water-based polyurethane.

Oops, I missed the part about wood grain showing through.

Water(or alcohol) soluable dyes from Transtint will do what you want.Most woodworking supply syores have the line or a similar brand(Woodcraft and Rockler) or you can order it on line.

Those dyes will only show vibrant color when used on ash/birch/maple and other very light woods.

Here are some examples of the dyes on ash.

Here is a link that might be useful: exam,ples

"I'll be giving these away on a small scale, so I'm not concerned about the CPSIA"

There is no exception for "giving these away."

The only way to skirt the law so far is to make only one toy and give it away. If you make two, under the current situation, your toys fall under the CSPIA.